ParallelVigraRfLazyflowClassifierFactory: Don't hard-code number of forests to use.
See ilastik/ilastik#1022
classifiers: All ClassifierFactory subclasses must have a VERSION class member, and must be a true subclass of LazyflowVectorwiseClassifierFactory or LazyflowPixelwiseClassifierFactory.
(This helps clients determine whether or not an unpickled classifier is compatible with the current source code.)